I was elated to see you are thinking of setting up a decentralized exchange. I've followed the progress of, say, B&C, but my perception is that they have become semi-centralized as they progress from vision to reality. Any interface with fiat will always be a choke-point, imho. I hope you solve it.
I think all 'decentralized exchanges' so far, and there are quite a few of them now, are semi-centralized - even where they use only crypto let alone with fiat.
A truly decentralized exchange between crytocurrencies would be too slow and probably too expensive in fees to be particularly useful for most people even if atomic cross chain transfer tech was built, so its a tough problem.
I also really hope a good solution can be worked out for this.
If you know anyone who could help with coding, my idea was to replicate DASH, set the masternode requirements to the extreme, yet only set the collateral for about 10 DEX (Decentralized Exchange Muney) and utilize all of the great features DASH uses for governance and mining to secure and grow the network.
I don't think speed would be an issue once we got the ball rolling. We could use the mechanics Evan laid out in Masternodes:
https://en.wikipedia.org/wiki/Oracle_machine,
https://en.wikipedia.org/wiki/Random_oracle#Applications,
https://en.wikipedia.org/wiki/Black_box_groupTo create a signed at random authorized order book - chain. We could create multiple side chain like derivatives -- which could all be based off Dash.
The idea is we could use the new Evolution DAPI to access this new exchange. It would initially only trade DASH/DEX through Evolution and this offshoot Masternoding network.
In time with funding and development, we could launch this into a more accessible exchange, and maybe start this as a project where the Dash foundation owns a particular percentage of the new project, similar to others proposals I have seen.